atris commented on a change in pull request #815: LUCENE-8213: Introduce
Asynchronous Caching in LRUQueryCache
URL: https://github.com/apache/lucene-solr/pull/815#discussion_r324497573
##########
File path: lucene/core/src/java/org/apache/lucene/search/LRUQueryCache.java
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@@ -89,12 +92,30 @@
*/
public class LRUQueryCache implements QueryCache, Accountable {
+ /** Throw this error when a query is asynchronously being loaded in the cache
+ * and a read for the same is requested
+ */
+ private static final class AsyncQueryLoadInProgressException extends
RuntimeException {
+
+ /**
+ * Sole constructor.
+ */
+ public AsyncQueryLoadInProgressException(String query) {
+ super(query);
+ }
+ }
+
private final int maxSize;
private final long maxRamBytesUsed;
private final Predicate<LeafReaderContext> leavesToCache;
// maps queries that are contained in the cache to a singleton so that this
// cache does not store several copies of the same query
private final Map<Query, Query> uniqueQueries;
+ // Marks the inflight queries that are being asynchronously loaded into the
cache
+ // This is used primarily to ensure that multiple threads do not trigger
loading
+ // of the same query in the same cache. We use a set because it is an
invariant that
+ // the entries of this data structure be unique.
+ private final Set<Query> inFlightAsyncLoadQueries =
Collections.newSetFromMap(new ConcurrentHashMap<Query,Boolean>());
Review comment:
We add to this map without a lock when triggering a caching operation, hence
the need for a concurrent safe set. The idea is that it is a better idea to let
two threads request an asynchronous cache load for the same query concurrently
than syncing the entire operation under a lock. However, this means that two
threads might want to add the same key to `inFlightAsyncLoadQueries` in the
same duration, so we need that operation to be idempotent and thread safe.
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